1. Field of the Invention
The present invention relates to continuous flow ink jet printing devices and, more particularly, to a device for effectively starting and stopping the flow of ink from the jet drop orifices.
2. Prior Art
Ink jet printing apparatus are fairly well known as is shown, for example, by Beam U.S. Pat. No. 3,577,198 and Mathis U.S. Pat. No. 3,701,998. Such devices include an ink reservoir behind a series of orifices through which ink or other printing liquid is ejected under pressure so as to produce a series of fine streams of droplets. As the droplets break from the main stream issuing from the orifices they pass through charging electrodes and receive a desired charge and subsequently pass through deflection electrodes which adjust the trajectory of the drops as desired to produce a dot matrix pattern on a printing medium or to be caught in a catcher before they impact the medium.
One of the problems associated with such devices is in starting and stopping the flow of liquid from the orifices. The printing operation cannot be initiated until a steady state condition is reached in the issuance of droplets from the orifices, and likewise, during shut down printing must be discontinued in advance of the stopping of the flow of ink from the orifices.
The nonsteady state flow of liquid from the orifices produces several problems in addition to the inability to print during that period. If the flow from the orifices is not initiated fast enough the liquid begins to weep from the orifices and wet the surrounding surfaces as well as form off-center droplets which drop and wet the charging and deflection electrodes, all of which can produce shorting or other electrical problems. Some of the ink which has wept from the orifices remains on the orifice plate and as it evaporates leaves a residue which after build up can eventually affect operation of the apparatus.
On shut down, several problems can also occur if the pressure is not decreased rapidly enough. The same large droplets can form as pressure slowly decreases and results in the same wetting of the orifice plate surface, charge electrodes and deflection electrodes. Also, if a negative pressure occurs the ink is drawn up into the orifices and, in turn, draws air into the reservoir behind the orifices which requires the additional step of purging the reservoir prior to the next startup.
One device which has been utilized in an attempt to overcome the above-described problems is a three-way directional control solenoid valve which is sufficiently fast acting to shut off and start up the flow of ink so as not to produce the undesirable results mentioned. However, such a solenoid valve requires the use of a magnetic steel armature which cannot be made sufficiently corrosion resistant to prevent clogging of the orifices due to loss of rusted material in the ink supply. Additionally, circuitry is needed to try to increase the valve speed and the flat elastomeric seals utilized therein tend to cut and shed particles on the valve orifices and are not speed reproducible due to the "bedding in" action of these seals. As shut down speed is solely a function of the valve spring response time and is consequently slow, the spring force is necessarily small since a stiffer spring would produce a slow start up response time. Therefore, the spring force must be balanced in a compromised position so as not to overly effect either start up or shut down initiation time. Also, constant current to the valve during printing operations draws considerable power and tends to heat the ink flowing to the drop generator which in time tends to clog the valve with viscous ink residue.
Several other methods have been devised for attempting to overcome these problems, but a need still exists for a relatively simple and inexpensive method of achieving a solution to these problems.